1. Field of the Invention
The present invention relates to a thermal head, and more particularly to improvements of the material for the protective layer for covering the plural drive circuit elements used for heating and driving a heating resistance element row of a thermal head.
2. Description of the Prior Art
FIG. 2 is a sectional view of a typical thermal head 1, which is referred to in the explanation of the prior art below, as well as the embodiment described below. The thermal head 1 comprises a heat resistant substrate 2 possessing an electric insulating property made of ceramics such as aluminum oxide Al.sub.2 O.sub.3. On the heat resistant substrate 2, a heat reserve layer 3 made of glass or similar material is formed, as being extending in a band form in the direction vertical to the sheet of paper in FIG. 2. At one side in the direction crossing the longitudinal direction of the heat reserve layer 3, for example, silver paste is printed to form a thick film common electrode layer 4.
Covering the entire surface of the heat resistant substrate 2, a resistance element layer 5 made of tantalum nitride Ta.sub.2 N or the like is formed. On the resistance element layer 5, in the upper part of the thick film common electrode layer 4, a thin film common electrode 6 extending parallel to the thick film common electrode layer 4 is formed of aluminum or other metal material, by thin film technology such as sputtering and etching. Relating to the heat reserve layer 3, on the opposite side of the thin film common electrode 6, plural band-shaped individual electrodes 7 are formed in plural rows in the direction vertical to the sheet of paper in FIG. 2, and the resistance element layer 5 enclosed by these thin film common electrode 6 and individual electrodes 7 is composed as a heating resistance element row 8 made of plural heating resistance elements.
On the individual electrode 7, plural drive circuit elements 9 for heating and driving selectively the heating resistance element row 8 are disposed parallel to the array direction of the heating resistance element row 8. To the drive circuit elements 9, thermal printing data and various control signals are supplied through signal lines 10 formed simultaneously when forming the individual electrode 7 on the heat resistance substrate 2. The drive circuit elements 9 possess bumps 11 for realizing connection with the individual electrode 7 and signal lines 10, which are connected by face-down bonding.
To the signal lines 10, an external wiring substrate 12 is connected. The external wiring substrate 12 comprises a support film 13 made of synthetic resin material, and a circuit wiring 14 formed on the support film 13. On the other hand, the heat resistance substrate 2 is adhered onto a cooling plate 15, with adhesive, which is formed by press forming or die-cast forming of, for example, aluminum. On the cooling plate 15, the external wiring substrate 12 is fixed through a spacer 16 made of synthetic resin material. Covering the thin film common electrodes 6 and individual electrodes 7, a wear-resistant layer 17 is formed by thin film technology such as sputtering, and the plural drive circuit elements 9 are entirely covered in the array direction with a protective layer 18 made of epoxy resin or the like.
This conventional thermal head involved the following problems.
(1) The protective layer 18 is made of epoxy resin. The epoxy resin has a coefficient of linear expansion of 2.0.times.10.sup.-5 .degree. C..sup.-1, and the ratio b/a of the coefficient of linear expansion b of the epoxy resin to the coefficient of linear expansion a of the substrate 2 is 2.74, which is significantly different from 0.73.times.10.sup.-5 .degree. C..sup.-1 of the ceramics used to compose the heat resistant substrate 2. The epoxy resin has a relatively high modulus of elasticity of 1300 kg/mm.sup.2.
On the other hand, to form the protective layer 18, an epoxy resin is applied to coat the entire surface of the plural drive circuit elements 9, and is hardened by heating at, for example, 120.degree. to 150.degree. C. Afterwards, when cooling to ordinary temperature of, for example, 25.degree. C., due to the difference in the coefficient of linear expansion, the protective film 18 shrinks more than the heat resistant substrate 2, and what is more the modulus of elasticity of the protective layer 18 of the epoxy resin is relatively large, so that the heat resistant substrate 2 may be warped in the direction vertical to the sheet of paper in FIG. 1.
In such warped state, when adhering the heat resistant substrate 2 and cooling plate 15, the layer thickness of the adhesive layer becomes nonuniform along the array direction of the heating resistance element row 8, and heat dissipation from the heating resistance element row to the cooling plate 15 becomes nonuniform. Such nonuniformity causes uneven contrast and lowers printing quality in thermal recording. As a result, the working efficiency drops in relation to positioning in this adhesion step. Besides, in connection of the signal lines 10 and external wiring substrate 12, positioning precision is lowered, and the job efficiency drops.
To prevent such inconveniences, if attempted to straighten the warp of the heat resistant substrate 2 by external force after hardening of the protective layer 18, the peripheral edge of the protective layer 18 may be easily cracked because of the large modulus of elasticity of the protective layer 18. In the event of such crack, for example, water may invade into the protective layer 18 to corrode the individual electrodes 7, or due to progress of corrosion, the individual electrodes 7 may be disconnected.
To avoid such problems, it is conventionally known to use a silicon resin for the protective layer 18. Such silicon resin is a so-called silicone rubber, and although its coefficient of linear expansion is also very different from that of the heat resistant substrate 2, it is relatively small in the modulus of elasticity. Accordingly, if a difference is caused in the contraction against the heat resistant substrate 2 due to temperature drop after forming the protective layer 18, the protective layer 18 is deformed elastically so that the heat resistance substrate 2 will not warp.
When using such silicon resin, however, since the elasticity is low, the hardness is low, and it is easily deformed when an external force is applied in the manufacturing process or during use, and the drive circuit elements 9 may be broken. Accordingly as in the conventional thermal head 1a in FIG. 1, a head cover 25 to cover the entire vicinity of the protective layer 18 is needed, and the number of parts increases, and the manufacturing process is complicated. The head cover 25 incorporates an elastic pressing member 27 inside of a groove 26 at a position opposite to signal lines 10, and the head cover 25 is fastened to the cooling plate 15 with a screw 28 through external wiring substrate 12 and spacer 16, and the external wiring substrate 12 is pressed and fixed to the signal lines 10.
Such protective layer 18 may contact with, in actual use, thermal paper 19 or transfer film, and in the case of silicon resin, the coefficient of friction is large when contacting, and paper jamming is likely to occur.
As described herein, the protective layer 18 of the conventional thermal head 1 is made of epoxy resin or silicon resin, and these resins involve the problems as mentioned above.